The pituitary gland is attached by a stalk to the region in the base of the brain known as the hypothalamus. In particular, follicle stimulating hormone (FSH) and luteinizing hormone (LH), sometimes referred to as gonadotropins or gonadotropic hormones, are released by the pituitary gland. These hormones, in combination, regulate the functioning of the gonads to produce testosterone in the testes and progesterone and estrogen in the ovaries, and they also regulate the production and maturation of gametes.
The release of a hormone by the anterior lobe of the pituitary gland usually requires a prior release of another class of hormones produced by the hypothalamus. One of the hypothalamic hormones acts as a factor that triggers the release of the gonadotropic hormones, particularly LH, and this hormone is referred to herein as GnRH although it has also been referred to as LH-RH and as LRF. GnRH was isolated and characterized as a decapeptide some 20 years ago, and it was found shortly thereafter that analogs of GnRH having a D-isomer instead of Gly in the 6-position, such as [D-Ala.sup.6 ]-GnRH (U.S. Pat. No. 4,072,668) having the following formula: EQU pGlu-His-Trp-Ser-Tyr-D-Ala-Leu-Arg-Pro-Gly-NH.sub.2,
have greater binding strength to the receptor and greater biological potency than the native hormone.
Peptides are compounds which contain two or more amino acids in which the carboxyl group of one acid is linked to the amino group of the other acid. The formula for [D-Ala.sup.6 ]-GnRH, as represented above, is in accordance with conventional representation of peptides where the amino terminus appears to the left and the carboxyl terminus to the right. The position of each amino acid residue is identified by numbering the amino acid residues from left to right. In the case of GnRH, the hydroxyl portion of the carboxyl group of glycine at the C-terminus has been replaced with an amido group(NH.sub.2) i.e. the C-terminus is amidated. The abbreviations for the individual amino acid residues above are conventional and are based on the trivial name of the amino acid, e.g. pGlu is pyroglutamic acid, Glu is glutamic acid, His is histidine, Trp is tryptophan, Ser is serine, Tyr is tyrosine, Gly is glycine, Leu is leucine, Nle is norleucine, Orn is ornithine, Arg is arginine, Har is homoarginine, Pro is proline, Sar is sarcosine, Phe is phenylalanine, Ala is alanine, Val is valine, Nva is norvaline, Ile is isoleucine, Thr is threonine, Lys is lysine, Asp is aspartic acid, Asn is asparagine, Gln is glutamine, Cit is citrulline, and Met is methionine. Except for glycine, amino acids of the peptides of the invention are of the L-configuration unless noted otherwise.
There are reasons for desiring to suppress gonadotropin secretions and to prevent ovulation in female mammals, and the administration of GnRH analogs that are antagonistic to the normal function of GnRH have been used for these purposes. For this reason, analogs of GnRH which are antagonistic to GnRH have been investigated for their potential use as a contraceptive, for regulating conception periods, as well as for the treatment of infertility, for the controlled induction of ovulation in women with chronic anovulation, and in in vitro fertilization. They are also useful for the treatment of precocious puberty, endometriosis (including endometriosis with pain), acne, amenorrhea (e.g. secondary amenorrhea), uterine myoma, ovarian and mammary cystic diseases (including polycystic ovarian disease (PCO)) and breast and gynecological cancers. The GnRH antagonists can also be useful in the symptomatic relief of the premenstrual syndrome (PMS). In the female, they can also be used to treat ovarian hyperandrogenism and hirsutism. Such antagonists have also been found useful to regulate the secretion of gonadotropins in male mammals and can be employed to arrest spermatogenesis, e.g. as male contraceptives for treatment of male sex offenders, and for treatment of prostatic hypertrophy. More specifically, GnRH antagonists can be used to treat steroid-dependent tumors, such as prostatic and mammary tumors, and for the control of the timing of ovulation for in vitro fertilization.
The GnRH antagonists can also be used to treat patients suffering from AIDS, rejuvenating the thymus when administered at a dosage of about 10 micrograms/kg/day to 1 mg/kg/day. The thymus then produces T-cells to replace the T-cells destroyed by the AIDS virus, thereby compensating for the effects of the virus.
There are a number of peptides that are known to cause histamine to be released from mast cells which cells are found in the skin, the gingiva and other locations throughout the body. As a result, inflammation is caused, often resulting in edema of the face and elsewhere on the skin. It was earlier found that certain GnRH antagonists that were effective in preventing ovulation had the undesirable adverse side effect of stimulating histamine release, generally rendering such GnRH analogs unacceptable for administration to humans. As a result, the design of GnRH analogs was directed to providing peptides that would retain the biological efficacy but would not have such undesirable histamine release, see J. Rivier et al., J. Med. Chem., 29, 1846-1851 (1986). In addition, it is important that the peptide analog should be well tolerated by the body upon administration, particularly when injected. Preferably, they also have good duration of action upon LH secretion, a property which is considered to be enhanced by resistance to proteolytic enzyme degradation in the body; thus, these properties are also kept in focus in designing new GnRH analogs. In addition, so as to facilitate administration of these compounds to mammals, particularly humans, it is considered advantageous for such GnRH decapeptides to have a high solubility in water, particularly bacteriostatic water, at normal physiologic pH, i.e. from about pH 5 to about pH 7.4.
In J. Rivier et al., J. Med. Chem., 35, 4270-4277 (1992), the design and synthesis of GnRH antagonists having improved properties in various of these respects are described. Despite the attractive properties of these GnRH analogs, the search has continued for still further improved GnRH antagonists, particularly ones that are well-suited to administration by subcutaneous injection and ones that are less expensive to manufacture. All of the GnRH antagonists of interest have at least three D-isomers in their amino acid sequence. Improved GnRH agonists are also desired.